The public is becoming increasingly aware of the dangers of surface soil contamination from a variety of sources, including chlorinated solvents, pesticides, chemical and microbial warfare agents, heavy metals, radioactive wastes, etc. The standard method for remediating such contaminated, near surface soils is excavation followed by incineration. Unfortunately, this is very expensive and requires removal and incineration of a large amount of soil in order to destroy very small concentrations of contaminants.
In Assignee's U.S. Pat. No. 4,984,594, Vinegar and Stegemeier propose a soil remediation method using an electrical surface heating blanket to heat the soil. The electrical heater, positioned on the soil surface, is permeable to vapors which emanate from the soil when heated. A permeable, thermally-insulating mat is located above the electrical heater, and an impermeable sheet is placed on top of the mat. A vacuum is established through a hole in the impermeable sheet through which vaporized contaminants are drawn as the soil is heated by the electrical heater.
In a preferred embodiment of the U.S. Pat. No. 4,984,594, the electrical heater comprises a mesh of electrically conductive metal wires which mesh is electrically energized by passing current through the wires. Instead of a mesh, the wires may also loop back and forth and may be stitched into a ceramic fiber cloth. The heater could also consist of metal strips or ribbons, thin metal foil or sheet, or a metallic coating in a defined pattern. Several of these designs were tried in small scale applications. Although these heater designs were effective in small scale applications, it proved difficult to scale up the system to a rugged, high-temperature field design that could be dragged over rough terrain. Another problem was that local irregularities in the soil, such as stones or holes, would result in local hot spots in the heater mesh. This caused rapid corrosion and resulted in destruction of the electrical heater, particularly when temperatures approached 1000.degree. C. When this happened the entire heater mesh had to be replaced. All the heater designs described in U.S. Pat. No. 4,984,594 consisted of fine mesh or other thin metal heating elements. The elements failed to distribute the heat from localized hot spots and were susceptible to thermal runaway.
In Assignee's copending application Ser. No. 800,192 filed Nov. 27, 1991, a plurality of stranded nichrome wires are placed inside an electrically insulating sleeve of Nextel. The sleeve is surrounded by a stainless steel tube, the assembly comprising a heating element. A plurality of heating elements are connected in a rigid frame to form a heater array having an electrical bus bar for supplying power to each heating element.
It is an object of the present invention to provide a surface heating blanket assembly that has several advantages relative to the designs disclosed in U.S. Pat. No. 4,984,594. Although the heating blanket disclosed herein is sturdier than previous designs, it is still lightweight and flexible, capable of being rolled up and easily transported by commercial vehicle to another site. At the site it may be dragged over terrain into position. The heater assembly conforms to ground contours over long distances. It has good mechanical strength and high temperature corrosion resistance up to at least 1000.degree. C. The heater achieves uniform areal temperatures as a result of its good two-dimensional thermal conductivity, which equalizes hot spots that might develop due to poor soil contact, surface irregularities, etc. The heater is also highly permeable to fluid flow and provides uniform air flow both horizontally and vertically. The heater is low cost and constructed from readily available commercial products. It is easily scaled up to very large areal dimensions. The heater also is easy to clean, free of moisture buildup, and has no tight places to trap even small amounts of contaminated soils. The heater has easily replaceable heating elements and may include redundant elements to allow continued operation if some elements fail during a remediation process. The heater has been found to be reliable in rugged terrain because the tensile stresses during dragging or rolling are not transmitted to the heating elements. Finally, the heater is intrinsically safer because the heating elements are completely sheathed with electrical insulation and current flow in the soil is eliminated.